The present invention relates to a unit for dewatering dredged material. More particularly the present invention relates to a vibrating screen unit for dewatering dredged material that can be used under unclean material conditions without reducing a feed rate to the machine used during clean material conditions.
Dredges dig and remove material, such as sand and gravel, from the bottom of bodies of water, such as lakes and quarries which have filled with water. The primary means for dewatering the dredged material is typically a vibrating screen unit having a deck surface covered by a screen mesh media that has a matrix of openings of a predetermined size and arrangement. Unlike conventional vibrating screen applications where material separation is achieved by having as much material as possible of a size less than that of the openings in the screen media fall below the screen deck surface, dewatering of the dredged material is achieved by retaining as much of the material above the size of the openings as possible on the screen deck surface while passing the water through the openings. During clean material conditions water and a small amount of fine grit pass through the openings in the screen mesh media while the dewatered material is discharged onto a conveyor belt for processing at a discharge end of the unit.
Dredges often encounter unclean material conditions where sludge, slime, and muck are retrieved along with desirable material. Sludge, slime, and muck do not easily pass through the openings in a screen mesh media of a vibrating screen unit. Therefore, most sludge, slime, and muck is discharged onto the conveyor belt along with the desirable material. Sludge, slime, and muck is semi-liquid and therefore does not move well on conveyor belts. The fluid nature of sludge, slime, and muck are particularly problematic when inclined conveyor belts are used because sludge, slime and muck tends to fall backwards, down an incline conveyor, where it collects and falls off. Over time, this sludge, slime, and muck accumulates and overflows back into the body of water from which it was retrieved. This results in it being brought up again by the dredge and reprocessed by the vibrating screen unit. In addition to this inefficient use of a dredge and vibrating screen unit, unclean material conditions make separation of sludge, slime, and muck from the sought after material, such as sand and gravel, difficult and more time consuming.
Current means available to handle unclean material involve a reduction in a feed rate of a vibrating screen unit to allow more time for the sludge, slime, and muck to be forced through the openings in the screen mesh media. Reduction of feed rate results in a reduced production rate for a dredging process to obtain clean material. A vibrating screen unit that solved the above problems associated witch unclean dredged material conditions without reducing the feed rate of the unit would be a welcome improvement.
Accordingly, the present invention includes a vibrating screen unit for dewatering dredged material. The vibrating screen unit includes a frame and a top screen deck that is coupled to the frame. The top screen deck has a first mesh size, a feed end that receives dredged material, and a discharge end that discharges substantially dewatered material of a size above the first mesh size. The vibrating screen unit also includes a middle screen deck coupled to the frame below the top screen deck that receives dredged material from the top screen deck. The middle screen deck has a second mesh size and a discharge end that discharges substantially dewatered material of a size above the second mesh size. The vibrating screen unit further includes a bottom screen neck coupled to the frame below the middle screen deck that receives dredged material from the top screen deck. The bottom screen deck has a third mesh size and a discharge end that discharges substantially dewatered material of a size above the third mesh size. A splitter of the vibrating screen unit is positioned to lie below the top screen deck to control the flow of material below the first mesh size to the middle and bottom screen decks. The splitter thus increases the screen deck surface area without increasing the floor space taken up by the unit. In preferred embodiments, the bottom and middle screen decks each have openings of a first size and the top screen deck has openings of a second size that are larger than the first size.
A carrying pan is positioned to lie below the middle screen deck and above the bottom screen deck to carry away water, grit, sludge, slime, and muck that passes through the openings of the top and middle screen decks. A "pant-leg" chute can be positioned to lie adjacent the discharge end of the middle screen deck to carry the contents of the carrying pan away from the unit. Water, grit, sludge, slime, and muck which passes through the openings in the bottom screen deck falls through the bottom of the unit.
In preferred embodiments, the top screen deck is angled in a range of between approximately five to seven degrees relative to a base of the frame such that the feed end of the top screen deck is above the discharge end of the top screen deck. In this preferred embodiment, the middle screen deck is also angled approximately 0.5 degrees relative to the base such that an end of the middle screen deck generally opposite the discharge end of the middle screen deck is above the discharge end of the middle screen deck. The bottom screen deck, however, remains generally parallel to the base.
Predetermined portions of the middle and bottom screen decks adjacent the discharge ends of these decks may be pivoted a predetermined number of degrees from in-line positions to vary the travel rates of material across these decks. In preferred embodiments, the predetermined number of degrees is approximately three degrees so that the middle and bottom screen decks can be pivoted through a total range of approximately six degrees. Pivoting the middle and bottom screen decks affects the rate at which material is conveyed across the middle and bottom screen decks which changes dewatering efficiency. For example, during unclean material conditions, the middle and bottom screen decks may be pivoted upward by up to approximately three degrees to slow down the flow rate of the dredged material in order to allow more water and impurities to fall through the openings in the decks.
The vibrating screen of the present invention includes a water treatment unit for clearing dredged material during unclean conditions. The water treatment unit is located on at least one screen deck. In preferred embodiments the water treatment unit includes one or more troughs that are located in one or more of the top, middle, and bottom screen decks. A spray bar is positioned to lie relative to each trough or row of troughs on a screen deck so that water from the spray bar is directed into the trough or a row of troughs to immerse the dredged material in churning water to help separate sludge, slime, and muck from the dredged material. The troughs may be integrated into a screen deck or may be fabricated as a separate component that is placed between sections of the screen deck. The troughs may be as deep as desired and, in preferred embodiments, extend from side to side of the frame of the unit in rows. The churning water helps cut up and liquify the sludge, slime, and muck so it falls through the openings in the screen deck under the influence of gravity more easily. When processing clean material, it is unnecessary to activate the spray bars. The troughs are preferably located towards the feed end of the unit to allow for a sufficient length of dewatering surface area on each screen deck after the material leaves the trough.
The vibrating screen unit includes a plurality of spring support assemblies that help to dampen vibration of the unit. Preferred embodiments of the unit, include four spring support assemblies, two positioned to lie below a feed chute of the frame and two positioned to lie adjacent the discharge end of the bottom screen deck. Incline adjustment pads may be located under each support spring assembly to allow for field adjustment of the incline of the entire unit by adding or removing pads from underneath either or both the spring support assemblies below the feed chute or the spring support assemblies adjacent the discharge end. The spring support assemblies each include a pivot trunnion assembly that allows the spring assembly to pivot when the incline of the unit is changed so that these assemblies remain substantially perpendicular with the support on which the unit rests. Chaning the incline of the unit changes the rate of travel of material through the unit.
The splitter includes a matrix of screen panels with a predetermined number of openings and solid panels that are both arranged in a pattern to control the quantity of material that passes to the bottom screen deck. Increasing the number of screen panels increases the amount of material that travels to the bottom screen deck whereas increasing the number of solid panels decreases the amount of material that travels to the bottom screen deck.
Flow dams may be positioned at various points on one or more of the screen decks. These dams extend generally perpendicularly from each deck, from side to side of the frame, so that they are transverse to the flow path of the material across the decks. The dams provide an obstacle which causes a small buildup of material on a screen deck surface which tends to compress the material thereby,forcing more water from it and enhancing the dewatering performance of the unit. These dams may be integrally cast or molded to the screen decks and may also be adjustable so that the height can be varied according to material conditions. In preferred embodiments, a dam is positioned at a predetermined location on the top screen deck adjacent the feed end of the top screen deck. The purpose of this dam is to cause as much material as possible of a size less than the openings of the top screen deck through these openings onto the splitter for division between the middle and bottom screen decks.
The vibrating screen unit includes a reversible counterweight that is coupled to a shaft counterweight of the unit to produce two different centrifugal forces based upon the direction of rotation and relative positions of the counterweights. During unclean material conditions, the reversible counterweight and shaft counterweight are rotated in a first direction where they are positioned relative to one another such that the mass of the counterweights add to produce a centrifugal force that causes the unit to vibrate with a particular amplitude. During clean material conditions, the direction of rotation of the counterweights is reversed so that the counterweights are positioned relative to one another such that the masses of the counterweights subtract to produce a smaller centrifugal force that causes the unit to vibrate with a smaller amplitude. Vibrating the unit with a larger amplitude increases material stratification on the top, middle, and bottom screen decks so that smaller material is forced towards the bottom of the beds of material on each of the decks. This forces a greater quantity of material below the first, second, and third mesh sizes through the respective top, middle, and bottom screen decks, thereby providing more opportunity for water and other impurities to be forced through the openings in these screen decks.
The frequency of vibration of the unit can be adjusted using any one of a number of means such as electronic variable frequency drives or hydraulic motors. Both examples provide a nearly infinite vibratory frequency range. Adjustment of the vibratory frequency along with changes in the vibratory amplitude allow for extensive adjustment of both unit speed and stroke to optimize those quantities for given material conditions.
In certain dredging operations where fine material such as sand is being retrieved, it is unnecessary for the vibrating screen unit to include a top screen deck. However, this embodiment of the vibrating screen unit includes the above-described structure, such as the splitter, water treatment unit, and carrying pan described above for the three screen deck embodiment of the unit. In this alternative embodiment of the unit, the two screen decks preferably have openings of a substantially equal size.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.